106 MICROSCOPIC OBJECTS. 



instrument shows the last class properly, it may he at 

 once pronounced superlative. So difficult are some of 

 these, that I do not know of half-a-dozen engiscopes or 

 microscopes that will exhibit them satisfactorily. 



"There are generally some very easy scales and feathers 

 among samples even of the most difficult kind. I must 

 therefore strongly impress upon the observer the necessity 

 of a careful selection. And here I may notice that the 

 darker the specimen the easier it is made out, and, in 

 general, the black ones are no proofs ; while, on the 

 other hand, the more transparent the tissue, the greater 

 the difficulty there is in developing its structm-e. Another 

 point I shall dwell upon is their proportions, or the 

 length and breadth of the object; for in some cases the 

 narrow long specimens are very difficult, while the short 

 broad ones are very easy. 



" The study of the manner in which these subjects are 

 exhibited is also of paramount importance ; for in pro- 

 portion to the excellence of the instrument will the dark- 

 ness and blackness of the Hues be increased and the 

 transparency of the spaces between them augmented ; 

 therefore, in comparing two instruments of the like con- 

 struction on the same object and under similar illumina- 

 tion, I should say that which shows the lines blackest 

 and the spaces most transparent is the best. Iii this 

 comparison I assume as a matter of course that their 

 magnifying powers are to be equal. The instruments 

 should also be of the same optical construction, or the 

 experiment will be unfair, for I have observed that in 



